I'm very well versed on the trans-jargon and I'm not even sure what they're asking for.
I'm guessing transwomen who have, in the past, provided the sperm to conceive children and so now "identify" as "mothers".
They could conceivably mean actual women who have given birth - but want to exclude women who have given birth who identify as male - ie transmen. It wouldn't be the first time transmen have been completely excluded from everything 
But as Penny Morduant - that champion of females - has said, mother has a specific legal definition, (apparently), in that it means the person who has given birth. Since no transwoman ever has given birth, the theatre can't mean transwomen.